In an optical communication field, an optical transmitter, which outputs an optical signal by modulating light in response to data to be transmitted, is used. In order to emit the optical signal being modulated, the optical transmitter includes a light source for emitting light, a light modulator for modulating light, and the like. The light transmitter is used for an optical transmission apparatus or the like for WDM (Wavelength Division Multiplexing) using multiple wavelength channels. The optical transmission apparatus for the WDM may include multiple optical transmitters for outputting optical signals each having a different wavelength, and an optical multiplexer for outputting a WDM optical signal in which multiple different optical signals output from the optical transmitter are multiplexed.
Japanese Laid-open Patent Publication No. 2001-83473 discloses a light modulator in which a optical waveguide for conducting light modulation is set so that an electric field which causes absorption of the electric field is smaller than an off-electric field of a modulation by a refractive index control, thereby an operation voltage is further decreased.
Also, Japanese Laid-open Patent Publication No. 06-232721 discloses a method and a device for monitoring optical switching node performance. A low-frequency signal is modulated on a light signal propagated along an optical transmission line at the input of an optical module. Optical modulation is carried out by adding a modulation voltage to a control voltage applied to the control electrode of a switching node in the optical module. The frequency is so selected sufficiently outside the bandwidth of a main light signal transmitted along the optical path. The amplitude of the modulated voltage is held minimum so that the level of the modulated light signal does not fluctuate so much. Consequently, it is guaranteed that information carried with the main light signal is neither destroyed nor disturbed.